Recently, the portable and cordless trend is rapidly promoted in electronic appliances such as AV appliances and personal computers. As the driving power source for these electronic appliances, there is a mounting demand for secondary batteries having high energy density in spite of small size and light weight. In particular, the nonaqueous electrolyte secondary battery with negative electrode using lithium as active material is highly expected as a battery having high voltage and high energy density.
In this battery, the active material for the positive electrode is a metal oxide containing lithium showing a voltage in a class of 4 V such as LiCoO.sub.2, LiNiO.sub.2, or LiMn.sub.2 O.sub.4, and the negative electrode is a carbon material which is capable of intercalating and deintercalating lithium.
The electrolytic solution to be used in such nonaqueous electrolyte battery is, for example, a nonaqueous solvent such as ethylene carbonate, propylene carbonate or other cyclic ester, tetrahydrofuran or other cyclic ether, dimethoxy ethane or other noncyclic ether, dimethyl carbonate, methyl propionate or other noncyclic ester, or a mixed solvent thereof.
Lately, the secondary batteries are expected to be used in various applications, not only in portable data terminals (for example, cellular phones and notebook type personal computers), but also in electric vehicles and power storage. Accordingly, applications in severe environments are increasing. In particular, when used as the power source for an electric vehicle, the battery may be exposed to high temperature of over 80 in summer, and the battery having a high reliability even in such severe ambient temperature is demanded.
In these secondary batteries, accordingly, a nonaqueous organic solvent not electro chemical decomposed at high voltage is used as the electrolyte solution. Such batteries are particularly heightened in oxidation catalytic function of the positive electrode in high temperature environment. As a result, the nonaqueous solvent is electro chemical oxidized on the surface of the positive electrode, the composition of the electrolyte solution is changed, the discharge characteristic deteriorates due to drop of conductivity of the electrolytic solution, and decomposition products (for example, carbon dioxide) are formed as gas. In a worst case, consequently, leakage of electrolytic solution occurred.
It is hence an object of the invention to present an excellent battery having a high reliability even when stored at high temperature.